travis: use default xenial instead of trusty

[daltonbohning]

The "trusty' distro seems to be missing a public key.

The Travis documentation recommends not using Trusty, as it is EOL:

• https://docs.travis-ci.com/user/reference/trusty/

For reference, this is the difference:

• https://docs.travis-ci.com/user/reference/xenial/#differences-from-the-trusty-images

Signed-off-by: Dalton Bohning daltonx.bohning@intel.com

[daltonbohning]

@adammoody Very simple change. This fixes the build errors that have been occurring, since trusty is EOL.